'

4 THE INTERNAL COMBUSTION ENGINE [CHAP, i

compression. A further difference is that the Lenoir engine
completed its cycle of operations in two strokes, and is known
as a " two-stroke " engine, whilst the Otto engine is a " four-
stroke " one. The advantage of compression is that the gases
are at a fairly high pressure before the ignition point is reached,
and so the effect of the explosion is to cause the mixture to
reach a far higher pressure, and therefore to do more work, than
if the pressure before explosion were no higher than that of the
atmosphere outside the engine. The fact that all engines work
on compression means that all must have a space provided into
wliich the piston can compress the charge. This space is called
the " clearance space." In Fig. 2 the face of the piston, at the
end of compression, comes up to the line AB, and on the expan-
sion stroke moves out as far as EF. Then the space between
AB and CD is called the " clearance " ; the distance from AB
to EF the " stroke " ; and the ratio of the volumes CDFE to
CDBA the " compression ratio," denoted by the letter r. It
is obvious that the higher the compression ratio the higher will
be the pressure at the end of compression, and thus the higher
will be the temperature at that point.

FIG. 2.—Diagram of Cylinder and Piston, showing clearance.

~ . .. volume G E F D

Compression ratio = _-___.., - H.

volume O A B D

6. Dugald Clerk.—In 1880 Dugald Clerk invented an
engine which partook of the nature of the Lenoir in that it
was a two-stroke engine, and of the nature of the Otto in that
the mixture was compressed before explosion. This he did by
mixing and slightly compressing the gas and air in a separate
cylinder instead of in the working cylinder. The working